The present invention relates to spot-beam satellite communications systems and, more particularly, to improvements in such systems afforded by the use of an antenna reflector and feed arrangement designed to provide multiple narrow beams, each capable of reusing the same spectrum.
In order to achieve greater communications capability, a number of multiple access satellite communications systems have been devised, one of which, described in U.S. Pat. No. 3,928,804, is known as space division multiple access. In such a system, a plurality of beams, commonly known as "spot" beams, illuminate specific areas of the earth's surface. Since the beams do not overlap, the same frequency band may be used for each beam, thus permitting increased use of the available bandwidth. In order to satisfy increasing communications demand, it is necessary to illuminate a large number of discrete areas from a single satellite. Since dedicating a separate reflector to each illuminated area would result in a very large and cumbersome arrangement of reflectors which would be difficult to deploy, a number of systems have been designed to which a common reflector is shared by a plurality of feeds. The multiple feeds may be arranged to direct radiation at the reflector from different angles so that each feed will illuminate a different area on the earth.
The sharing of a common reflector by a plurality of feeds does not, in itself, provide a complete solution to the above-mentioned problem--i.e., illuminating a large number of discrete areas from a single satellite. Reflector and feed arrangements of the type used until now have only been capable of satisfactorily illuminating one-half of the earth's hemisphere visible from the satellite, and, thus, more than one reflector is needed. In addition to problems encountered in deploying the multiple reflectors, the reflectors may differ in size or be unequally spaced around the satellite, thus causing imbalances which may significantly decrease the useful life of the satellite. For example, the torque imparted to the satellite by the sun's rays may cause an increase in fuel consumption to maintain the orientation of the satellite. Also, the heat generated by the absorption of the sun's rays may be greater on some parts of the satellite than others, thus causing thermal stresses in the satellite structure.
Some systems, e.g., ATS-6 satellites, utilize symmetrically illuminated paraboloidal reflectors. However, in these systems, the feeds are positioned within the radiating aperture of the reflector, and large side lobes occur in the far-field radiation pattern due to the blockage of the radiating aperture.
Other systems, e.g., ANIK, INTELSAT IV-A and INTELSAT V satellites, utilize offset-fed paraboloidal reflectors in which each of the feeds illuminates a section of the reflector offset from the vertex of the parabola. In these systems, the feeds are not located within the radiating aperture, and, consequently, no aperture blockage occurs. However, these systems do exhibit the above-mentioned disadvantage of being effective over only one-half of the earth's visible hemisphere. This is due to the increase in aberration, or defocussing, of the radiated beam which occurs as the offset of the feed from the focus of the paraboloid is increased. The defocussing of the antenna beams also results in poor cross-polarization cancellation. This means that these systems are not particularly adaptable to frequency reuse systems employing orthogonally polarized antenna beams.